The present invention relates generally to improved techniques (as well as software and/or apparatus) for analyzing the data produced by measurement imaging tools of semiconductor features such as contact holes and trenches.
In the manufacturing of semiconductor computing chips, a variety of feature defects are impacting the yield of high performing computer chips. One example of this is the failure of holes to be cleared of resist after lithographic exposure and develop which can go undetected. Timely detection of this condition and related ones can allow the product to be saved by rework. Even in situations where rework is not an option, early detection of feature flaws can be important. As the printed dimensions shrink and the fidelity of printing is challenged by lower k lithography, detection of such defects becomes more challenging.
Most modern electric circuits are formed as xe2x80x9cprintedxe2x80x9d circuits on a substrate. In the field of circuit fabrication, one of the more critical functions is the ability to control the size (length, width, height), or critical dimensions (CD), of features such as contact holes and trenches which form the circuits. Any deviation in the dimensions of a feature can adversely impact on the performance of the resulting circuitry.
As critical features become smaller, metrology (the science of measurement) using imaging tools such as optical microscopes, scanning electron microscopes (SEM), atomic force microscopes (AFM), and other similar tools becomes more challenging. The challenge is especially acute for contact holes and trenches with diameters less than 0.25 xcexcm. For example, reduced collection efficiency of SEM secondary electrons from such contact holes and trenches can provide false edge sharpening. For this and other reasons, current measurement methodologies are poor in their correlation with cross section results taken from actual samples. Reduced collection efficiency also may interfere with obtaining critical information from the bottom of a contact hole.
U.S. Pat. No. 6,185,323, entitled xe2x80x9cMethod characterizing a feature using measurement imaging toolxe2x80x9d teaches the importance of analyzing the information obtained from a critical dimension imaging tool that lies between the hole feature edges. More specifically, U.S. Pat. No. 6,185,323 discussed fitting the data profile or waveform between the edge locations to a quadratic or trapezoidal mathematical form. The xe2x80x9cgoodness-of-fitxe2x80x9d of this fitting provides a metric to distinguish clearly open holes from clearly closed ones. This technique is also valuable for detecting other kinds of feature defects in other geometries (lines, spaces, etc). The disclosure of U.S. Pat. No. 6,185,323 is incorporated herein by reference.
Unfortunately, the technique of U.S. Pat. No. 6,185,323 often fails to have good discriminating power for holes with diameters less than about 180 nm. In the more general applications of this technique to other geometries, there are issues at small dimensions as well. Other conventional approaches such as (a) simple waveform correlation where a template is acquired from a xe2x80x9cgoldenxe2x80x9d wafer fail to provide sufficient discrimination capability, and (b) the conventional approaches discussed in the background of U.S. Pat. No. 6,185,323, are even more deficient in this regard.
The deficiencies of these methodologies show that a need still exists for a method which will accurately and reliably characterize a substrate features such as a contact hole or trench, especially as the dimensions of such features continue to shrink.
The invention provides improved analytical methods as well as software, system and apparatus implementing the improved analytical techniques. In one aspect, the invention encompasses use of a combination of weighting function(s) and correlation function(s) (e.g., mathematical forms, templates, etc.) in the analysis of feedback from structural features. The invention also encompasses the concept of a calibration database useful for determining a suitable combination of weighting and correlation functions, especially where the calibration database contains information relating to the actual performance or function of reference structural features. The invention also encompasses methods, apparatus, systems and software involving such calibration database and/or the use of a combination of weighting and correlating functions to characterize substrate features.
In one aspect, the invention encompasses a calibration database stored in a computer readable medium, the database comprising:
(a) information elements describing at least one functional performance characteristic of respective structural features on a substrate, and
(b) information elements describing feedback from the respective structural features as a function of position over each of the respective structural features.
The calibration database preferably further comprises (c) information elements describing physical analysis of each of the respective structural features. The feedback preferably is secondary electron emission from the structural features upon exposure to a scanning electron beam. The structural features are preferably holes in a resist layer a semiconductor substrate. The functional performance characteristic is preferably the response of each respective hole to an etching protocol.
In another aspect, the invention encompasses a method for evaluation of target structural features on a substrate, the method comprising:
(a) providing information elements describing feedback from the target structural features as a function of position over each of the respective target structural features on the substrate,
(b) applying a combination of a weighting function and a correlation function to the target structural feature information elements to predict a functional performance characteristic of respective target structural features and/or to describe a physical characteristic of respective target structural features.
Preferably, a functional performance characteristic, such as etchability across the target feature, is predicted in step (b). The structural features are preferably holes in a resist layer on the substrate. The information elements are preferably embodied in computer-readable media, and step (b) is preferably performed using a computer. The invention also encompasses systems, apparatus, and/or computer programs for carrying out the above method(s).
In another aspect, the invention encompasses, a method for evaluation of target structural features on a substrate, the method comprising:
(a) providing a calibration database comprising:
(i) information elements selected from the group consisting of (A) information elements describing a functional performance characteristic of respective reference structural features on a substrate, and (B) information elements describing a physical characteristic of each of the respective reference structural features, or both types of information elements, and
(ii) information elements describing feedback from the respective reference structural features as a function of position over each of the respective reference structural features,
(b) providing at least one weighting function as a function of position over each of the respective reference structural features and at least one correlation function as a function of position over each of the respective reference structural features, wherein a plurality of weighting functions and/or correlation functions is provided,
(c) determining a combination of weighting function and correlation function from the provided which provide a desired degree of correlation between the information elements (i) and (ii) for respective reference structural features,
(d) providing information elements describing feedback from the target structural features as a function of position over each of the respective target structural features on the substrate, and
(e) applying the combination of weighting function and correlation function to the target structural feature information elements to predict the functional performance characteristic of respective target structural features and/or to describe the physical characteristic of respective target structural features.
Preferably, a plurality of weighting functions and/or a plurality of correlation functions are provided in step (b). The weighting functions and/or correlation functions may be continuous functions and/or discontinuous functions. A value of the weighting function of the determined combination is preferably multiplied with a value of a respective information element in step (e). The calibration database is preferably one as described above. Preferably, the information elements are embodied in a computer-readable medium, and steps (c) and (e) are performed using a computer. The invention also encompasses systems, apparatus, and/or computer programs for carrying out the above method(s).
These and other aspects of the invention are described in further detail below.